1. Field of the Invention
The present invention relates to wires for arc welding, particularly, a flux-cored wire and a solid wire for arc welding, which are improved in arc stability by sticking molybdenum disulfide (hereinafter, referred to as MoS.sub.2) on the surfaces thereof, without provision of any plating.
2. Description of the Related Art
Solid wires used for welding mild steel in a protective atmosphere such as carbon dioxide or argon gas have been generally applied with copper plating on the surface layers thereof. These wires have been generally used by a method wherein, for example, a wire having a small diameter ranging from 0.9 to 1.6 mm is fed to a portion to be welded at a welding rate of from 1 to 20 m/min, and it is applied with a large welding current of from 80 to 500 A for melting the portion to be welded. Copper plating has been considered to be effective to improve wire feedability, to enhance current-carrying characteristic between a feeding point (welding tip) and a wire, and to improve rust resistance of wires during storage. Recently, from the viewpoint of operability, there have been strong demands towards continuous welding for a long term using robots and automatic welders, and to meet these demands, it has become further important to improve the wire feedability, current-carrying characteristic between a tip and a wire, and wear resistance of a tip.
However, it has been pointed out that even copper plating wires have an essential disadvantage in introducing arc instability and thereby increasing the amount of spatter. The reason for this is as follows: namely, copper plating is possibly separated from a wire due to the unevenness of the plating depending on manufacturing variations, and due to the friction between the wire and a conduit liner occurring in a drawing process after plating and during wire feeding, so that the base metal of Fe is exposed, thus obstructing the stable current-carrying.
To solve the above disadvantage, it may be considered to increase the amount of copper plating. However, if so, the copper plating is rather made easier to be separated; and it presents another disadvantages that during welding, hazardous fume containing copper and copper oxides is generated in a large amount, which is very undesirable in terms of the welding environment, and further, the amount .of copper in weld metal is increased, which leads to high temperature cracking.
Of the copper plating wires, there has been known a wire of a type in which an oxygen-rich layer is produced on the surface of the wire by adjustment of annealing temperature and atmosphere in manufacturing processes, to lower the surface tension of droplet upon welding, thereby improving arc stability and reducing the amount of spatter.
The above wire, however, is intended to refine droplet, and is not effective in improving current-carrying stability. Moreover, in general, the intergranular-oxidized wire is relatively large in surface irregularities in addition to the presence of the oxidized layer, so that the copper plating is rather made easier to be separated from the wire, which exerts adverse effect on current-carrying stability.
In view of the foregoing, it is essential to apply a suitable surface treatment in place of copper plating. In other words, the presence of copper plating on the surface of a wire is undesirable, and thus it is essential to eliminate the copper plating and to apply a suitable surface treatment capable of satisfying various characteristics.
Wires with no copper plating have been proposed, for example in Examined Japanese Patent Publication Nos. SHO 55-128395, SHO 53-6247 and SHO 53-3940, and Unexamined Japanese Patent Publication No. SHO 49-91950. These prior arts are intended to improve wire feedability, current-carrying stability between a tip and a wire, and rust resistance by coating MoS.sub.2, graphite, zinc dithiophosphate, lubricating oil, inhibitor and the like in a suitable amount on the surface of a wire.
The above techniques, however, are insufficient in improvement of wire feedability and current-carrying stability.
Unexamined Japanese Patent Publication No. HEI 6-262389 discloses a wire coated with organic molybdenum on the surface thereof for improving wire feedability. This wire is effective in wire feedability; but is not effective in current-carrying stability.
The above prior arts have also failed to essentially solve such a problem that the rust resistance of a wire with no copper plating is poor as compared with that of a copper plating wire.